Nearly all vehicular diesel engines have used turbochargers for many years and, more recently, are becoming more prevalent on gasoline engines due to their ability to improve fuel consumption. The more stringent miles-per-gallon regulations imposed by the federal government standards will be the motivation for more vehicle engines to be turbocharged.
Small turbochargers have become a viable commercial product, primarily due to the development of satisfactory bearing systems that have allowed them to operate successfully at very high rotational speeds. A very great amount of time and effort has been expended over the years to develop bearing systems that damp destructive shaft vibrations, insulate the rotor from external shock loads, and withstand the heat transferred into the shaft from the hot turbine wheels that are exposed to engine exhaust gas.
The most prevalent of these successful bearing systems utilize floating sleeve bearings to support the shaft and have an inner and outer oil film where the outer oil film provides a damping cushion that permits the turbocharger rotor to pass through its critical speed without destroying the bearing system. The floating sleeve bearings also permit the rotor to find and rotate about its mass center, thereby eliminating radial forces that would be imposed on the bearings if the rotor were constrained to rotate about its geometric center.
Dynamic balancing of the rotor components and the rotor assembly results in making the mass center and geometric centers coincidental, however, there is almost always a small difference left between these centers in practice, and that causes an orbital motion of the rotor. This orbital motion is permitted to occur within the oil film thicknesses of the floating sleeve bearings and contributes to the long-term durability of the floating sleeve bearing systems.
The early floating sleeve journal bearing systems required a separate thrust bearing, capable of carrying the axial thrust forces generated in turbocharger operation that can occur in both axial directions. Since the friction loss in radial thrust bearings that are perpendicular to the shaft axis is proportional to the fourth power of the radius, any collar attached to the shaft that bears axially against the stationary thrust bearing surface will generate a relatively high friction loss. Accordingly, the radius of the thrust bearing should be kept as small as possible in designing a complete turbocharger bearing system.
An early attempt to minimize thrust bearing losses is illustrated in U.S. Pat. No. 3,390,926, dated Jul. 2, 1968, where a large shoulder on the turbine wheel hub bore against the end of a tubular one piece bearing, the other end of which was forced against a stationary plate to carry thrust in one axial direction. The tubular bearing was rotatably carried on a film of oil in the stationary bearing housing and had two axially spaced journal bearing surfaces on its inside diameter. When rotor conditions caused thrust in the opposite direction, a collar attached to the shaft was forced against the said stationary plate to carry the thrust load.
This bearing system worked satisfactorily as long as exhaust gas temperatures were moderate. However, when exhaust gas temperatures became high as with highly rated engines, the heat carried from the turbine wheel to the hub that bore against the end of the tubular bearing could cause distress on the bearing surface.
Sleeve bearing systems are particularly desirable in turbochargers which employ wide usage because they are less expensive to manufacture and can be easily assembled. However, there is a need for improved sleeve bearing systems which are more efficient and reliable when operating at very high speeds in the presence of very high temperatures, such as those experienced by turbochargers for highly rated internal combustion engines.